Theta solvent
A solvent is referred to as a theta solvent (or also θ solvent ) if a polymer dissolved in it behaves like an ideal chain .
Polymers in theta solvents show, as well as polymers in the melt , a d -dimensional random walk behavior, wherein d the dimensionality of the subject polymer.
Physical interpretation
The conformation assumed by a polymer chain in dilute solution can be modeled as a random walk of the monomer subunits using the model of the freely connected chain . However, this simple model does not take steric effects into account .
The behavior of real polymer is better due to the self-avoiding path (English self-avoiding walk, SAW ) described as conformations, in which different segments of the polymer chain occupy the same space, are not physically possible. This excluded volume causes the polymer to expand.
The conformation of the polymer chain is influenced by the quality of the solvent. The intermolecular interactions between polymer chain segments and solvent molecules lead to an interaction energy which can be positive or negative. If the polymer has good solubility in the solvent, interactions between polymer segments and solvent molecules are energetically favorable and cause the polymer to expand. For poor solubility, on the other hand, polymer-polymer interactions are preferred and the polymer contracts. The solubility depends on the chemical composition of the polymer and the solvent as well as on the temperature of the solution.
If a polymer is just so poorly dissolved in a solvent that the effects of volume exclusion are canceled out, the theta (θ) condition is met. The theta condition is fulfilled for a given combination of polymer and solvent at the so-called theta (θ) temperature . A polymer solution at theta temperature is called a theta solution.
In general, the properties of polymer solutions depend on the solvent. However, if a theta solution is used, the measurable properties are independent of the solvent used. The polymer then behaves exactly as it is predicted by the ideal chain model. This considerably simplifies the experimental determination of important quantities such as the end-to-end vector or the radius of gyration.
Individual evidence
- ↑ a b c Gert R. Strobl: The Physics of Polymers- Concepts for Understanding their Structures and Behavior. Springer-Verlag, Berlin / Heidelberg 1996, ISBN 3-540-60768-4 .
- ^ A b Michael Rubinstein, Ralph H. Colby: Polymer Physics. Oxford University Press, New York 2003, ISBN 0-19-852059-X , pp. 101f.